342 H. B. ANDERVONT 



to develop tumors was either a unique characteristic of these hybrids or the 

 strain BALB/c females carried a latent or masked virus; the latter possibility 

 appeared likely in view of the susceptibility of these mice to the strain C3H 

 virus. A series of investigations was performed to reveal the presence of a 

 mammary tumor virus, or a variant therefore, in strain BALB/c mice. These 

 studies are not reviewed because a publication by Andervont and Dunn 

 (1953) contains all relevant references. It was concluded that the virus was 

 not responsible for the high incidence of tumors in old hybrids obtained from 

 BALB/c females and that these tumors resulted from the interplay of 

 hormonal stimulation and hereditary factors in the absence of the virus. 



A deliberate attempt to mask and then unmask the virus has been made 

 with the virus-resistant C57BL strain. These mice were suckled by strain 

 C3H females and by suitable procedures it was found that the virus had 

 disappeared from them within a few generations (Andervont, 1945c). 

 Descendants of these mice have been maintained for more than twenty 

 generations of inbreeding and repeated efforts to expose the virus in them 

 have been unsuccessful. 



The virus appears to be very stable once it is established within an inbred 

 strain, but the literature does contain a few references to its sudden appear- 

 ance or disappearance. Bittner (1941) described the occurrence of a mammary 

 tumor in a strain A mouse whose ancestry for seven generations did not 

 develop this type of tumor but whose descendants for four generations showed 

 a high incidence of tumors. Bittner suggested that the milk of mice may 

 contain an inactive virus which may change to an infectious virus or the 

 infectious virus may arise de novo within a mouse. Murray and Warner 

 (1947) watched for and observed the disappearance of the virus from a 

 mouse of the Marsh strain. They had observed previously a progressive 

 decrease in the tumor incidence of the strain and had attributed it to the 

 presence within the strain of a family which did not develop tumors. Starting 

 with a pair of mice whose ancestry showed a high incidence of tumors, they 

 observed that neither one of the daughters nor any of her 351 female descen- 

 dants developed a breast tumor. After analyzing all the available data, the 

 conclusion was reached that the Marsh strain females transmitted variable 

 amounts of virus, or an altered virus, and the variation in the quantity or 

 quality of virus remained consistent through later successive generations. 

 This interesting paper has been overlooked by most reviewers of the mammary 

 cancer virus. 



For many years the writer has maintained inbred strains of mice and has 

 watched carefully for any evidence of contagion and for the sudden appearance 

 of the virus. Strains free of the virus but highly susceptible to it have been 

 housed close to strains which carried it and the former were observed 

 constantly for any sudden acquisition of the virus. This has not occurred but, 



